Yet more on pterosaur quad arm posture

In a message dated 6/27/13 3:26:25 AM, mark.witton@port.ac.uk writes:
<< Pterodactyloid trackways show that their limbs are held in parasagittal
poses, and those of azhdarchids seem to drop almost vertically from the
body, as depicted in my 2008 illustration and subsequent works. I'm not the
only person to illustrate and describe their postures in this way: Chris
Bennett (1997), David Unwin (1997, 2005), Hwang et al. (2002), Mazin et al.
(2003) and many others have done the same. Presumably, the skewed joint at
the end of the metacarpal deflects the distal wing laterally when the arm
is folded up. Thus, I'm happy that my reconstructions are more-or-less
consistent with this idea. The idea that pterodactyloids had sprawled
forelimbs, akin to the Paul (2011) reconstruction of Q. n., is not
supported by any trackway data.
>>
This is a remarkable statement. Including how citing a list of persons as
doing a particular thing is useless if it is none of them or others have
backed up with proper documentation. And as is becoming increasingly clear they
have not come close to doing.
Do not know what the "Paul (2011) reconstruction of Q. n." is refering to.
Aside from not remembering publishing a pterosaur illustration that year, I
restore azhdarchids with narrow trackways.
There seems to have arisen a strange opinion that a narrow trackway
precludes a sprawling humerus. This is of course long been known to be
incorrect,
including the Gambaryan & Kuznetsov paper this year in J Zool illustrating
the combination of a sprawling humerus and narrow trackway in echidnas. And
that in an animal in which the humerus makes up around half the arm length. In
azhdarchids the radius-ulna+metacarpal length ratio is about 4. So with the
humerus sprawling, just flex the elbow about 95-100 degrees ventrally and
medially and you get as narrow a trackway as you want. Duh. Seriously, this
is obvious, what is this argument about? If Witton etc saw an echnidna
trackway they would insist it is erect limbed, but it ain't.
To determine limb posture trackways are important, and when the trackway
gauge is very wide it must be sprawling, but when it is narrow joint
articulations etc must be examined to determine humerus posture.
We know quad dinosaurs had erect arms not just because of their narrow
trackways, but because the shoulder glenoid faces predominantly
ventro-posteriorly, with the scapula component often facing a little medially,
precluding a
sprawling humerous posture (as I have published in the technical
literature). It is not an exactly parasagital gait, the elbow is bowed out some
(as is
common in mammals), and the hands are more widely separated than is common
in mammals. But it is erect, not sprawling.
The primary function of the super specialized pterosaur arms was flight.
That was what they were mainly adapted for. Ground locomotion was a secondary
function that had to be accomodated to the flight adaptations.
We have no doubt that the pterosaur humerus could sprawl because that is
what it had to do in flight, and becasue the shoulder glenoid faces laterally
as in sprawlers, not postero-ventrally. So far no one has shown that any
pterosaur could position its humerus in an erect posture. Vague armchair
assertions that a laterally facing shoulder joint will also allow an ungulate
posture will not do. Until someone publishes a detailed diagram showing it can
be done, or mounts a cast based on well preserved bones -- anyone want to
point out an example? -- it is very dubious speculation at best, and very
likely wrong.
No to "the "skewed joint at the end of the metacarpal deflects the distal
wing laterally when the arm is folded up." The flexion at the joint of the
base of the wing finger had to operate in the horizontal plane during flight,
so the outer wing could be swept back during say diving flight. If the wing
finger base acted as W suggests then sweeping the outer wing strongly back
during flight would also cause it to arc upwards, partly flat on to the air
flow, which would be aerodynamic madness. That's why nothing like that
happens with the outer wing of birds. Wellnhofer 1991 says something obscure
about
the wing finger base joint causing the outer wing to twist for folding the
outer wing so its top side faces laterally, but that does not match his own
figure on the previous page, or on p 137.
Take a look at that last figure. And p 156 of the Wellnhofer books. Those
quad poses get the articulations of the joints and orientations of the wing
elements right -- except for the short fingers. Humerus. Sprawling (as also
in flight, same as in birds). Humerus condyles for radius and ulna. Directed
ventrally (so rest of wing can be downstroked in flight at elbow because
humerus cannot depress much below horizontal -- the lack of down flexion in
Wellnhofer 87 p 153 is way off, as is the flexion at the wrist -- also
streamlines joint, same as in birds). Radius anterior and maybe a little medial
to
ulna (presents minimal profile to airflow when flying, same as in birds).
Slender metacarpals anterior to stout one (also maximizes streamlining by
keeping leading edge of hand directed anteriorly as in birds). Folded outer
wing
directed posteriorly to clear body, and in same plane as
radius-ulna-metacarpus (basically same as in birds).
Some of you may be bouncing off walls saying but the trackway gauge is wide
in the figures. Stop, calm down, and think about it. Be Zen. Note how the
wide hand gauge is because the humeri are directed somewhat dorsally (as
during the beginning of the flight upstroke), while the elbow flexion is about
90 degrees, so the arms slope down and out, placing the hands laterally for
no particular reason. Here's how to solve the problem. Simply depress the
humerus about 20 degrees until it is horizontal, and/or flex the elbow more as
needed, swinging the hands medially until they are nearly on the midline.
There you go, a sprawling humerus with a narrow trackway. Ta-da! While at it,
rotate the humerus along its long axis so the rest of the arm is more
vertical rather than directed strongly forward as shown in both figures, and it
is
ready to walk, baby.
Also change the orientation of the fingers. They are shown directed
forwards, but that cannot be correct because the distal line of the small
metcarpals runs antero-posteriorly (this is like trying the wrench the fingers
of
your left hand hard to the right, on the same plane as the metacarpus and 90
degrees relative to their true extension-flexion arcs - ouch!). With the
fingers correctly extending dorsally relative to the plane of the metacarpals
they will splay laterally.
Here's another way to look at it. Take the two sprawling quad figures. Keep
the radius-ulna-metacarpus complex articulation and orientation realtive to
the body exactly the same. But depress the humerus a little until it is
horizontal. And extend the wing finger. You now have the flight downstroke
(with the elements presenting minimal frontal profile). Which we know worked
just fine (do the same with the bones from your handy dandy bird wing, they
have the same articulations and orientations in the downstroke). Now, refold
the wing finger, and you have the basic quad pose, except flex the elbow a
little more (remember, it can flex until the radius-ulna are nearly folded
against the humerus as in birds and us, so getting the walking posture is easy)
to get that narrow trackway we all know and love. Neat, isn't it. All that
has to then be done to walk on the arm is to rotate the humerus on the
shoulder joint (presumably rolling it along its long axis [also used to vary
the
wing's angle of attack relative to the body in birds and pterosaurs according
to Wellnhofer] and perhaps fore and aft horizontally some [used to help
vary wing sweep]) to achieve a stride, pretty long because the
radius-ulna+metacarpal is so long so only a little motion of the humerus will
do it. There
is little action at the elbow. It's all so easy, no fuss, no bother, using
the same basic bone posture and muscle actions as in flight for quad walking,
no strange if not impossible articulations or rotations. Because the same
muscles are used for walking as for flapping, no need to have distinct walking
versus flying muscles that add weight.
If we instead try swinging the humerus back approaching 90 degrees (in
dorsal view) into an ungulate posture, aside from probably disarticulating the
shoulder joint, the outer wing is going to be directed right into the body,
unless there is the improbable rotation at the elbow or along the long axis
of the radius-ulna that look locked together along their shared plane (these
are not feasible in birds, chances are low that pterosaurs could do them)
that is undocumented arm chair speculation. The erect arm idea is directly
self contradictory. The wing finger cannot collapse less than about 25 degrees
tight on the metacarpus. So the more tucked in the eblow, the more the outer
wing jabs into the body. And what for? Why have the arm for some mysterious
and illogical reason going all erect what with the outerwing banging into
the body and adding weight for muscles that work the humerus fore and aft and
what all. That when keeping the humerus parsimoniously exploits the flight
posture and avoids also those problems. Anatomically probably impossible and
does not make functional sense.
Take a gander at the photo of the complete Santanadactylus wing p 125
Wellnhofer. It shows the arm in flight posture in dorsal view. If the winger
finger is swept back nearly 90 degrees it will remain in the same plane,
pointing directly down in the photo, no? The photo also shows the arm in close
to
posterior view if the humerus is assumed to be in an erect walking posture
(particularly the ulna-radius-metacarpus, there would be some flexion of the
elbow). So what will happen to the wing finger when it is folded? It pokes
right smack into the middle of the body, doesn't it. Anatomical nonsense,
agreed? And there is no way to swing the folded outer wing outwards to clear
the
body by radius-ulna rotation or other joint rotations in such a locked up
system. How to get the outer wing to fold posteriorly? Easy. Flex the elbow
about 90 degrees, swing the humerus laterally until it is sprawling, and the
folded outer wing is clear of the trunk, right?
Regarding Bennett's 97 restoration of pterowalking. It's wrong. Bill says
he did not attempt to restore the actual joint action in the paper, so his
posture is intuitive not scientific. The first clue that something is not
right is that with the humerus erect the wing finger is folding on the
outside(!) of the arm, which is opposite of what would actually happen. Why the
180
degree error? Because the proximal end of the radius in both pterosaurs
figured is shown articulating with the ulna condyle of the humerus. (There is
no
way to get the left radius so far medial with the left humerus in the erect
posture shown. Get your bird bones and try it. To get the radius in
something close to that position the humerus has to be nearly horizontal and
directed very strongly anteriorly, with the distal humerus condyles directed
medio-ventrally posteriorly, all of which is impossible. In the figure the left
elbow is also hyperextended well beyond the articulations. To get the right
radius in the exact position and orientation shown requires having the humerus
directed nearly vertically upwards. I'm not kidding, get your bird wing
bones and ye shall see.) If the proximal ends of the radius and ulna are
articulated properly the outer wing is directed inwards and banging into the
belly. Also incorrect is the articulation of the short fingers, they are
rotated
90s degrees relative to their actual position on the metacarpus so the palm
faces medially as in the trackways. With the orientation of the radius, ulna
and metacarpus as shown in the fingers, the fingers would be splayed
anteriorly. To actually get the fingers splayed out laterally as per the
trackways
the radius needs to be anterior to the ulna, with the slender metacarpals
leading the stout one. That happened automatically with, and only with, the
humerus sprawling. Also incorrect is the orientation of the wing finger
relative to the metacarpus. In the view shown in the figures we should be
seeing
the leading edge directed laterally, towards the viewer. Instead it is
directed posteriorly (which gives the illusion of proper outerwing folding when
walking), and that would again actually happen only with the radius and
slender metacarpals on the leading edge during walking which requires a
sprawling
humerus (in case you have not gotten the message yet).
The fact is that most folks have not been paying into to the details. Some
have posed the wing elements pretty much properly and have the humerus
sprawling as it should be, but ignored the trackways so the elbow is too
extended
and the hands separated too far (a common flaw of quad mounted skeletons).
Many have been paying hardly any attention to how the joints articulate and
how the arm must work as a wing and assuming a narrow trackway demands an
erect humerus when a little geometry and some modern animals proves
otherwise. I would do something about this, but it is a major project that
would
include obtaining casts of the Santanadactylus arms (cannot even get the
Leonardi & Borgomanero 87 paper throught the JHU system). What should happen is
that I should be the reviewer of all papers on the subject of archosaur
articulation since I have been at this so long and have a better ability to
mentally visualize functions in 3-D than most it seems (have already shot down
a
couple of bad articulation papers in peer review for major bio journals). That
would eliminate a lot of nonsense. Seriously, send the manuscripts to me
and I'll determine if they make sense or not.
So if one wishes to continue to put ungulate posed arms on pterosaurs, get
out there and show how it actually works with nifty detailed diagrams
including of the joint articulations and/or a mounted cast. With the joints
actually articulated as they should be. Preferably in the technical literature.
And when it does not work out and you instead publish a paper showing the
humerus was sprawling as the hands were nearly on the midline, then be sure to
credit my correct illustrations. There has been way too much failure to
credit priority of late.
By the way, as noted by Wellnhofer pterosaurs could not elevate the humerus
vertically as is normal in modern flying birds. It is very likely that most
if not all pterosaurs had good climbing flight abilities. So the inability
of basal dinosaurian fliers like Archaeopteryx and Microraptor to fully
elevate th humerus does not necessarily refute their ability to be good powered
fliers. Contemplate it.
GSPaul
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